Various methods have been proposed that measure sleepiness and wakefulness level of a subject. The most reliable method to measure sleepiness and wakefulness level uses brainwaves or information of the blood flow in the brain. This method, however, imposes a great burden on the subject and therefore have not been practically used outside hospitals and research related facilities.
On the other hand, other methods use a heart beat (pulse) signal of a subject to measure sleepiness and wakefulness level of the subject without imposing a burden on the subject. For example, Japanese Laid-open Patent Publication No. 2004-350773 discloses a technology that evaluates sleepiness level of a subject depending on whether the frequency of the heart beat signal of the subject is lower than a reference value, i.e., a peak frequency of the heart beat signal in a wakeful state. Japanese Laid-open Patent Publication No. 08-299443 discloses a technology that evaluates the sleepiness level of a subject based on the strength of a low frequency portion and a high frequency portion of a heart beat signal, or based on the ratio of the low frequency portion to the high frequency portion.
The conventional technologies, however, pose a problem that the sleepiness level of the subject cannot be determined stably with the use of the heart beat signal.
In Japanese Laid-open Patent Publication No. 2004-350773, sleepiness is determined based on the band of a peak frequency of the heart beat signal. However, the heart beat signals differ among individuals and the peak frequency of the heart beat signal widely fluctuates depending on physical conditions of the subject. Thus, sleepiness level cannot be evaluated stably.
In Patent Japanese Laid-open Patent Publication No. 08-299443, the sleepiness level of the subject is evaluated based on the ratio of the low frequency portion to the high frequency portion of the heart beat signal. Because the borders between the low frequency and the high frequency differ among individuals and the fluctuation depending on the physical conditions is large, the technology cannot be used widely. Further, as to the low frequency portion and the high frequency portion, the low frequency portion is extremely large in power. Thus, if the ratio between these two portions is taken, an error in the low frequency portion greatly affects the evaluation of sleepiness level, and thus stable evaluation cannot be realized.
Thus, it has been a challenge to stably determine the sleepiness level of a subject without being affected by physical conditions of the subject or differences among individuals.